In surgeries requiring a total hip joint replacement, the upper portion of the femur must be replaced and an appropriate material must be selected for each modular replacement component. Over the past 25 years, a number of replacement materials have been identified for use as the femoral head component. In particular, modular metallic and alumina head components have been extensively studied. However, the metallic components are highly susceptible to corrosion and the alumina components typically have a flexural strength of only about 600 MPa. The low strength of alumina heads is particularly problematic for 22.22 mm diameter designs, as they historically have failed to safely exceed the FDA goal that the head be able to withstand a rupture load of 46 kN.
Because of its superior flexural strength of at least 900 MPa, yttria stabilized zirconia polycrystal ("YTZP") has been selected by the medical community for use in artificial hip joint prosthesis heads, and has even been used in 22.22 mm heads to attain rupture load of at least 60 kN.
Although 22.22 mm YTZP heads have attained the rupture load desired by the FDA, they have generally only done so when fitted with metallic stems having diameters of only 8 mm to 10 mm. Unfortunately, the medical community routinely uses much larger diameter stems (e.g., the Euro-cone having a "12/14" diameter stem) during hip joint surgery. One very popular stem is the "Euro-cone", whose frustoconical portion has a 12 mm end which enlarges to a 14 mm end at a total angle of about 6 degrees (typically between about 5 37'37" and 5 43'30"). Since modularity and interchangeability are valued attributes in hip joint replacement surgery, and the 22.22 mm zirconia head binds the surgeon to an unconventionally small diameter stem, surgeons have been reluctant to adopt these heads. Accordingly, widespread use of the 22.22 mm zirconia head has been limited.
Another concern in designing hip joint prosthesis heads is the ability to tailor the overall length of the head-trunnion combination to the needs of the particular patient which can only be determined at the time of surgery. In modern prosthesis systems, flexibility in length is provided by holding the trunnion length constant and designing a set of same diameter ceramic heads to slide upon the trunnion stem with varying degrees of recess penetration. The heads within a particular set often produce differences in overall head-trunnion length of at least 3 mm, and are typically identified as "short neck", "medium neck" or "long neck" heads. The "short neck" head of a set has a deep, wide cavity designed for deep stem penetration and takes its name from the short length of trunnion neck 84 visible after stem penetration. See FIG. 1a. Conversely, the "long neck" head of a set has a shallow, narrow cavity for shallow stem penetration and takes its name from the long length of trunnion neck visible after stem penetration. See FIG. 1c.
Therefore, there is a great need to develop a kit of short, medium and long neck 22.22 mm zirconia heads differing by at least 3 mm in neck length which can fit on a standard 12/14 stem and attain rupture loads of at least 46 kN.
The strength of a volume of material is typically determined by the size and frequency of its intrinsic and extrinsic flaws.
Intrinsic flaws are those found throughout the microstructure of the material and typically include agglomerates, inclusions and porosity. Intrinsic flaws may be in the volume of the material or on its surface. Extrinsic flaws are typically introduced by machining and grinding damage, and so are found only at or near the surface of the material.
The tensile strength of YTZP zirconia is known to be a function of flaw size and the degree of yttria stabilization. Noguchi et al., J. Am. Cer. Soc. 73 (9) 2667-76 (1990), found the mean tensile strengths of pressureless sintered YTZP's to range from 430 MPa to 550 MPa and hot isostatically pressed ("hipped") YTZP's to range from 570-745 MPa.
When the stress upon a particular volume of ceramic material exceeds the tensile strength of that volume, the material ruptures. Therefore, in designing a ceramic hip joint prosthesis head, it is helpful to understand the stresses which act upon the head and the factors which determine the strength of the head. When the recess of a head is press fit onto the metallic stem, the metallic stem undergoes a plastic deformation and pushes laterally against the taper wall of the recess. Two types of tensile stresses develop in the ceramic material from this action. The first is a hoop stress which acts outwardly upon ceramic material ringing the stem. The second is a crown stress which acts upon the upper corners of the recess and is caused by the directionally opposing hoop stresses on opposing sides of the taper wall. Hoop stresses are also present at the upper corners of the head recess, often reaching over 80% of the associated crown stress.
Fessler, Proc. Instn. Mech. Engrs. Vol 203 Part H, 1989, pp.15-31, extensively studied the stresses in conventional heads and found it reasonable to expect that the maximum tensile stress values would occur in the inner crown surface and not elsewhere.
Fessler characterized each head by outer radius Ro 87 and a centerline recess radius ro 86, and found that conventional heads typically possess ro/Ro values of between 0.30 and 0.47. See FIG. 1a. Fessler also taught an optimum ro/Ro value of about 0.37.